As an example, the present invention can be applied to the field of cellulose, and the need of the papermaking industry to form webs of fibrous suspensions, which are often more than 10 meters wide and have very high requirements for uniformity both across and along the manufacturing direction. The uniformity of the webs is often totally decisive for the efficiency and economy of the process.
During, for example, liquid treatment, i.e. washing or bleaching and dewatering of fibrous suspensions, it is essential that the pulp is applied to and distributed transversely on a continuous liquid permeable support as uniformly as possible, both in the longitudinal and transverse direction. It is equally essential that the treatment liquid is distributed uniformly across the pulp web. A uniform distribution prevents channelization and thereby non-uniform liquid treatment and dewatering.
In order to make optimum use of the equipment, a suspension of cellulose fibers to be dewatered, for example in connection with a liquid treatment, must be supplied at the highest possible concentration and be formed on the support, through which liquid is sucked out. This support can be a liquid permeable roll or a plane wire.
The question of highest possible concentration is determined by the capacity of the device to transversely distribute the medium sufficiently uniformly and to form a homogenous web in order to satisfy the demand of the process in question. The difficulty of distributing the fibrous suspension uniformly, across the entire width increases rapidly with increasing pulp concentration as a consequence of the increasing shearing strength of the fiber network.
Non-uniform distribution of the fibrous suspension not only results in non-uniform dewatering and thereby poor efficiency of the liquid treatment, it can also cause in a press damages on the fibers and, thus, deteriorated pulp quality. In the roll nip between the rolls, for example in a roll press, fiber flocks and thick portions of the pulp web can be subjected to compacting pressures so high that the fibers are damaged in these load-carrying portions.
It is thus apparent that a uniform distribution of the fibrous suspension and treatment liquid is of greatest importance for the quality of the final product and economy of the process.
One type of distribution unit used during the distribution of fibrous suspension is described in Swedish patent specification No. SE 500,546. The distribution unit in that case comprises a distribution housing with a wide outlet opening and a pipe for the supply of the suspension to the distribution housing. In the distribution housing the suspension is deflected and simultaneously spread in the lateral direction, and thereafter deflected again in a passage bent in the transverse direction. Such a distribution unit delivers a uniformly distributed and parallel flow.
In order to form a web of up to 10 meters width, and even greater width, several distribution units are required. Each distribution unit of the type described in Swedish patent specification No. 500,546 normally is about 0.9 m wide, but today they are found to have widths of up to about 2 meters.
For supply of the suspension to the distribution units, a head box is provided substantially along the entire width of the web. The head box acts as a large equalizing box, in which the pulp shall be uniformly distributed. The head box has an outlet for each distribution unit. From each outlet a pipe extends to its distribution unit.
With such a device of a known type, problems arise in that the pulp web becomes non-uniform as seen along the entire width of the web. Each distribution unit brings about a uniform web, but the different distribution units between themselves do not bring about exactly equal webs. Problems also arise because the distribution units at times can plug.
One object of the present invention is to offer a method and a device for uniformly spreading and distributing a flowing medium to a web, for example a web of pulp suspension, where the medium is distributed by means of at least two distribution units. As mentioned above, the uniformity of the web is fully decisive for the efficiency and economy of the process.
It has thus been found that the head box at times does not deliver the same flow in all outlets.
The flow distribution is achieved by static throttlings. During pulp flow a pressure drop is created above the throttling of the respective outlet. It is this pressure drop which produces the distribution of the pulp between the different outlets. Particularly during low flows the pressure drop created in the throttlings becomes too low for providing a good distribution. This, in turn, results in the different distribution units not being fed fully equally which, of course, results in a non-uniform web.
It is also due to the non-uniform distribution from the head box to the distribution units that the distribution units can plug.